This invention relates to a drive such as a chain, belt, band, or tape drive, with spaced noncircular, rotational members interconnected in driving relation by a flexible endless chain, belt, band, tape or the like. More particularly, the invention relates to a drive of this nature that accomplishes the desired functional relationship, and in which the noncircular members and elongate flexible drive member entrained about them in driving relation operate with a minimum of slack variation in the flexible drive member.
Over the years, chain drives with noncircular sprockets, particularly for bicycles, have been proposed. Many or all proposals of this nature lack any teaching of how to accomplish a preferred functional relationship between driving and driven noncircular rotational members. Many such proposals entirely fail to address the likely variations in slack in the chain and how to eliminate that or compensate for that. Some proposed drives with one slightly elliptical sprocket simply rely upon the conventional chain drive's limited tolerance for variation in slack. Other proposed noncircular drives of this nature utilize a movable compensatory slack take-up idler sprocket or roller to maintain the chain taut as the sprockets turn, by taking up the excessive slack that occurs. Another proposed approach has been to vary the distance between the sprockets during each rotation to take up the excessive slack that would otherwise periodically occur. On occasion, it has been suggested to make both of the pair of sprockets noncircular, but without any clear indication how this might be accomplished.
In this last category of chain and sprocket drives with two noncircular sprockets, one suggestion in the patent literature was to use on a bicycle two elliptical sprockets of differing sizes about which is entrained the typical chain. No consideration was given to the differing rates of rotation of the two different sized sprockets, or the resulting changing angular relationship between sprockets, whereby one sprocket would not necessarily be in the correct position to take up slack when the other tended to contribute to the slack. A similar proposal was to mount eccentrically the rear sprocket of a bicycle so that, as that sprocket turned, it was to move eccentrically about its axis of rotation to take up the slack that resulted from an elliptical drive sprocket. Again, there was no recognition that the changing angular relationship between the sprockets throughout the rotation thereof would prevent operation of the drive as desired.
Only simple functional relationships between driving and driven rotational members have been sought for the prior art noncircular drives mentioned above. Writings on the subject have expressed no understanding that accomplishment of a variety of functional relationships between driving and driven members could be sought while minimizing slack variations.